Reducing optical loss in reflective optical gratings

ABSTRACT

An optical device includes a light-transmitting medium on a base. The light-transmitting medium at least partially defines a free propagation region through which light signals travel. A reflective grating is positioned such that light signals can travel through the free propagation region and be received by the optical grating. The optical grating is configured to reflect the received light signal back into the free propagation region. The optical grating reflects the light signals such that light signals associated with different wavelengths separate as the light signals travel through the free propagation region. The portion of the light-transmitting medium that defines the free propagation region has a facet through with the light signals are transmitted. The grating includes a buffer layer between the facet and a reflecting layer that is configured to reflect the light signals received by the grating.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Patent application Ser. No. 61/284,723, filed on Dec. 23, 2009, entitled “Reducing Optical Loss in Reflective Optical Gratings,” and incorporated herein in its entirety.

FIELD

The present invention relates to optical devices and more particularly to optical devices used in communications applications.

BACKGROUND

Optical devices used in communications applications often have a light-transmitting medium positioned on a base. Optical gratings for separating light signals of different wavelengths are often formed in the light-transmitting medium. Waveguides for guiding the light signals can also be defined in the light-transmitting medium. The light-transmitting medium generally includes a free propagation region between the waveguides and the optical grating.

During operation of these optical devices, a portion of the waveguides carry the light signals to the free propagation region of the light-transmitting medium. The light signals travel through the free propagation region to the optical grating. The optical grating includes a reflecting surface that reflects the received light signals back into the free propagation region. The light signals are reflected such that the light signals of different wavelengths separate as they travel through the free propagation region. Another portion of the waveguides receive the separated light signals from the free propagation region and carry the separated light signals away from the optical grating.

These optical gratings are associated with an undesirably high level of optical loss. As a result, there is a need for improved optical gratings.

SUMMARY

An optical device includes a light-transmitting medium on a base. The light-transmitting medium at least partially defines a free propagation region through which light signals travel. A reflective grating is positioned such that light signals can travel through the free propagation region and be received by the optical grating. The optical grating is configured to reflect the received light signal back into the free propagation region. The optical grating reflects the light signals such that light signals associated with different wavelengths separate as the light signals travel through the free propagation region. The portion of the light-transmitting medium that defines the free propagation region has a facet through with the light signals are transmitted. The grating includes a buffer layer between the facet and a reflecting layer that is configured to reflect the light signals received by the grating.

Another embodiment of the optical device includes a light-transmitting medium on a base. The light-transmitting medium at least partially defines waveguides and a free propagation region through which light signals travel. The waveguides include an input waveguide and output waveguides. A reflective grating is positioned such that light signals guided by the input waveguide can travel through the free propagation region and be received by the optical grating. The optical grating is configured to reflect the light signal back into the free propagation region such that the light signals are received at the output waveguides. The optical grating reflects the light signals such that light signals associated with different wavelengths are received at different output waveguides. The portion of the light-transmitting medium that defines the free propagation region has a facet through with the light signals are transmitted. The grating includes a buffer layer between the facet and a reflecting layer that is configured to reflect the light signals received by the grating.

Another embodiment of the optical device includes a light-transmitting medium on a base. The light-transmitting medium at least partially defines waveguides and a free propagation region through which light signals travel. The waveguide include an input waveguide and output waveguides. A reflective grating is positioned such that light signals guided by the input waveguide can travel through the free propagation region and be received by the optical grating. The optical grating is configured to reflect the light signal back into the free propagation region such that the light signals are received at the output waveguides. The optical grating reflects the light signals such that light signals associated with different wavelengths are received at different output waveguides. The portion of the light-transmitting medium that defines the free propagation region has a facet through with the light signals are transmitted. The grating includes a reflecting layer that is configured to reflect the light signals transmitted through the facet. An upper portion of the reflecting layer is positioned on a top of the light-transmitting medium. The grating also includes a buffer layer between the upper portion of the light-transmitting medium and the reflecting layer.

Methods of fabricating the optical device are disclosed.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A through FIG. 1C illustrate an optical device that includes input waveguides configured to carry multiple light signals to an optical grating. The optical grating is configured to demultiplex the light signals. The device includes output waveguides configured to carry the demultiplexed light signals away from the optical grating. FIG. 1A is a topview of the device.

FIG. 1B is a cross-section of the device shown in FIG. 1A taken along the line labeled B.

FIG. 1C is a cross-section of the device shown in FIG. 1B taken along the line labeled C.

FIG. 1D illustrates the structure of reflecting surfaces included in the optical grating.

FIG. 1E illustrates one or more secondary cladding layers formed on the optical device of FIG. 1A through FIG. 1D.

FIG. 2A through FIG. 2C illustrate a portion of a device precursor upon which the waveguides and optical grating is formed. A first mask is formed on the device precursor. The first mask leaves exposed a recess region of the device precursor and trench regions of the device precursor. FIG. 2A is a topview of the device precursor.

FIG. 2B is a cross section of the device precursor shown in FIG. 2A taken along the line labeled B in FIG. 2A.

FIG. 2C is a cross section of the device precursor shown in FIG. 2A taken along the line labeled C in FIG. 2C.

FIG. 3A through FIG. 3C illustrate a second mask formed over the first mask on the device precursor of FIG. 2A through FIG. 2C. FIG. 3A is a topview of the device precursor.

FIG. 3B is a cross section of the device precursor shown in FIG. 3A taken along the line labeled B in FIG. 3A.

FIG. 3C is a cross section of the device precursor shown in FIG. 3A taken along the line labeled C in FIG. 3C.

FIG. 4A through FIG. 4C illustrate a third mask is formed on the device precursor of FIG. 3A through FIG. 3C after removal of the second mask. FIG. 4A is a topview of the device precursor.

FIG. 4B is a cross section of the device precursor shown in FIG. 4A taken along the line labeled B in FIG. 4A.

FIG. 4C is a cross section of the device precursor shown in FIG. 4A taken along the line labeled C in FIG. 4C.

FIG. 5A through FIG. 5C illustrate a cladding formed on the device precursor of FIG. 4A through FIG. 4C after removal of the third mask and the first mask.

FIG. 5A is a topview of the device precursor.

FIG. 5B is a cross section of the device precursor shown in FIG. 5A taken along the line labeled B in FIG. 5A.

FIG. 5C is a cross section of the device precursor shown in FIG. 5A taken along the line labeled C in FIG. 5C.

FIG. 6A through FIG. 6C illustrate a reflecting layer formed on the device precursor of FIG. 5A through FIG. 5C. FIG. 6A is a topview of the device precursor.

FIG. 6B is a cross section of the device precursor shown in FIG. 6A taken along the line labeled B in FIG. 6A.

FIG. 6C is a cross section of the device precursor shown in FIG. 6A taken along the line labeled C in FIG. 6C.

DESCRIPTION

The optical device includes an optical grating and a light-transmitting medium positioned on a base. Waveguides and a free propagation region are defined in the light-transmitting medium such that the free propagation region is positioned between the waveguides and the optical grating. During operation of the optical device, a portion of the waveguides carry the light signals to the free propagation region of the light-transmitting medium. The light signals travel through the free propagation region to the optical grating. The optical grating includes a reflecting surface that reflects the received light signals back into the free propagation region. The light signals are reflected such that the light signals of different wavelengths separate as they travel through the free propagation region. Another portion of the waveguides receive the separated light signals from the free propagation region and carry the separated light signals away from the optical grating.

The optical grating includes a reflecting layer that reflects the light signals back into the free propagation region. A suitable reflecting layer includes metals. Due to the inaccuracies in the process of manufacturing these devices, a portion of this metal is positioned on top of the free propagation region of the light-transmitting medium. The metal on top of the free propagation region absorbs light from the free propagation region and is accordingly a source of optical loss for the optical device. Further, since this optical loss is through the top of the free propagation region, this loss is a source of polarization dependent loss (PDL). In order to reduce this optical loss, the current optical device includes a buffer layer between the top of the free propagation region and the portion of the metal on top of the free propagation region. Due to the angle at which the light signals are incident upon this portion of the buffer layer, this portion of the buffer layer act as a waveguide cladding and can accordingly reduce interaction of the light signals with the metal on top of the free propagation region. As a result, this portion of the buffer layer reduces absorption of the light from the free propagation region and accordingly reduces polarization dependent optical loss associated with the optical device.

In one embodiment, the optical grating is an echelle grating. The reflecting surface of an echelle grating includes a series of stepped reflecting surfaces. Each of the steps includes a primary reflecting surface and a shadow surface. The primary reflecting surface are the surfaces that are designed to reflect the light signals such that the light signals separate and the shadow surfaces physically connect the primary reflecting surfaces. The metal is located on the primary reflecting surfaces but can also be positioned on the shadow surfaces. The metal on shadow surfaces can absorb incident light signals and is accordingly a source of optical loss for the optical device. Further, this loss is also a source of polarization dependent loss (PDL). In order to reduce this optical loss, a portion of the buffer layer can be located between the free propagation region and the portion of the metal on the shadow surfaces. This portion of the buffer layer can act as a waveguide cladding and can accordingly reduce interaction of the light signals with the portion of the metal on the shadow surfaces. As a result, this portion of the buffer layer reduces absorption of the light from the free propagation region and further reduces polarization dependent optical loss associated with the optical device.

The optical device can include a cladding over the light-transmitting medium in order to protect the waveguides. In some instances, this cladding can also serve as the buffer layer. As a result, an additional manufacturing step is not needed to form the buffer layer. Further, the buffer layer need not just be positioned on top of the free propagation region but can also be positioned over the facet of the free propagation region such that the buffer layer is between the facet of the free propagation region and the portion of the metal that reflects the light signals back into the free propagation region. As a result, when the cladding serves as the buffer layer, there is no need to remove the cladding from the facet of the free propagation region. Accordingly, the use of the cladding as the buffer layer can actually reduce the number of manufacturing steps associated with fabrication the optical device.

FIG. 1A through FIG. 1D illustrate an optical device. FIG. 1A is a topview of the device. FIG. 1B is a cross-section of the device shown in FIG. 1A taken along the line labeled B. FIG. 1C is a cross-section of the device shown in FIG. 1B taken along the line labeled C. FIG. 1D illustrates the structure of a reflecting surfaces included in an optical grating on the optical device. The device is within the class of optical devices known as planar optical devices. These devices typically include one or more waveguides immobilized relative to a substrate or a base. The direction of propagation of light signals along the waveguides is generally parallel to a plane of the device. Examples of the plane of the device include the top side of the base, the bottom side of the base, the top side of the substrate, and/or the bottom side of the substrate.

The illustrated device includes lateral sides 12 (or edges) extending from a top side 14 to a bottom side 16. The propagation direction of light signals along the length of the waveguides on a planar optical device generally extends through the lateral sides 12 of the device. The top side 14 and the bottom side 16 of the device are non-lateral sides.

The device includes a light-transmitting medium 18 positioned on a base 20. The portion of the base 20 adjacent to the light-transmitting medium 18 is configured to reflect light signals from the light-transmitting medium 18 back into the light-transmitting media in order to constrain light signals in the light-transmitting medium 18. For instance, the portion of the base 20 adjacent to the light-transmitting medium 18 can be an optical insulator 22 with a lower index of refraction than the light-transmitting medium 18. The drop in the index of refraction can cause reflection of a light signal from the light-transmitting medium 18 back into the light-transmitting medium 18. Suitable light-transmitting media include, but are not limited to, silicon, polymers, silica, SiN, GaAs, InP and LiNbO₃.

The base 20 can include an optical insulator 22 positioned on a substrate 24. As will become evident below, the substrate 24 can be configured to transmit light signals. For instance, the substrate 24 can be constructed of a second light-transmitting medium that is different from the light-transmitting medium 18 or the same as the light-transmitting medium 18. The illustrated device is constructed on a silicon-on-insulator wafer. A silicon-on-insulator wafer includes a silicon layer that serves as the light-transmitting medium 18. The silicon-on-insulator wafer also includes a layer of silica positioned on a silicon substrate 24. The layer of silica can serve as the optical insulator 22 and the silicon substrate 24 can serve as the substrate 24.

The device includes waveguides 26. Trenches 28 that extends part way into the light-transmitting medium 18 each includes one or more waveguide surfaces 30. Each waveguide surface 30 defines a ridge 32 that defines a portion of a waveguide 26. For instance, the ridges 32 and the base 20 define a portion of a light signal-carrying region where light signals are constrained within each of the waveguides 26. When the device is constructed on a silicon-on-insulator wafer, the silica that serves as the insulator 22 has an index of refraction that is less than an index of refraction of the silicon light-transmitting medium 18. The reduced index of refraction prevents the light signals from entering the substrate 24 from the silicon. Different waveguides 26 on the device can have different dimensions or the same dimensions.

A first cladding 33 is positioned on the waveguides. The first cladding 33 can have a lower index of refraction than the light-transmitting medium 18. The drop in the index of refraction can cause reflection of a light signal from the light-transmitting medium 18 back into the light-transmitting medium 18. If foreign materials such as dust or materials from fabrication of the device were able to directly contact the light-transmitting medium 18, these foreign materials could be a source of optical loss in the waveguide. As a result, first cladding 33 can protect the waveguide and can reduce the optical loss associated with these foreign materials. Suitable claddings 33 include, but are not limited to, silicon, polymers, silica, SiN, GaAs, InP and LiNbO₃. In some instances, the first cladding 33 material is the same material as the optical insulator 22. For instance, the optical insulator 22 and the first cladding 33 can both be silica.

The device includes an optical grating 34 such as reflection grating. A first portion of the waveguides 26 acts as input waveguide configured to carry a light beam that includes multiple light signals to the optical grating 34. A second portion of the waveguides 26 acts as output waveguides configured to carry light signals away from the optical grating 34. The first portion of the waveguide 26 can include one or more of the waveguides and the second portion of the waveguide 26 can include one or more of the waveguides.

The optical grating 34 is configured to receive a light beam that includes multiplexed light signals from one or more of the input waveguides. The optical grating 34 demultiplexes the light signals such that different light signals are received at different output waveguides. The device can be constructed such that only one of the demultiplexed light signals is received at each output waveguide or such that more than one of the demultiplexed light signals is received at one or more of the output waveguides. A suitable optical grating 34 includes, but is not limited to, reflection gratings. A particular example of a reflection grating is an echelle grating. The illustrated optical grating 34 is an echelle grating.

The grating includes a free propagation region 40 and a grating recess 42. The grating recess 42 is partially defined by surfaces that include an incident surface 44 defined in the light-transmitting medium 18. For instance, the optical device can include a grating recess 42 extending into the light-transmitting medium 18. During operation of the device, a beam from an input waveguide enters the free propagation region 40. The beam includes multiple light signals. The path of the light through the free propagation region 40 is illustrated as dashed lines in FIG. 1A in order to distinguish the light from other features of the device. The light signals travel through the free propagation region 40 and are incident upon the incident surface 44. Since the light signals can be transmitted through the incident surface 44, the incident surface 44 serves as a facet for the free propagation region.

The grating recess includes a buffer layer 45 between the incident surface 44 and a reflecting layer 46. The details of the relationship between the buffer layer 45, incident surface 44, and reflecting layer 46 is not evident from FIG. 1A but can be readily discerned from FIG. 1B and FIG. 1D. The light signals that are incident upon the incident surface 44 are transmitted through the incident surface 44 and then travel through the buffer layer 45 and are reflected by at a surface of the reflecting layer 46 as shown by the arrow in FIG. 1B. The surface at which an echelle grating reflects lights signals includes a series of stepped reflecting surfaces 47. The reflecting surfaces 47 are surfaces of the reflecting layer 46 and are at an interface of the reflecting layer 46 and the buffer layer 45. Suitable materials for the reflecting layer 46 include, but are not limited to, dielectrics, and metals such as Al and Au.

The reflecting surface 47 are configured such that light of different wavelengths separates as it travels away from the reflecting layer 46. Accordingly, the echelle grating demultiplexes the beam into individual light signals, or channels, traveling away from the reflecting layer 46. The channels are received at the output waveguides. FIG. 1A illustrates the output waveguides configured such that each of the channels is received on a different one of the output waveguides.

FIG. 1D illustrates the structure of the reflecting surfaces 47 included in the optical grating. The reflecting surfaces 47 each includes a primary reflecting surface 48 and a shadow surface 49. Although the primary reflecting surfaces 48 are illustrated as being substantially flat, the primary reflecting surfaces 48 can be a curve such as an elliptical curve.

As is evident from FIG. 1A and FIG. 1D, the primary reflecting surface are the surfaces that are designed to reflect the light signals such that the light signals separate. The shadow surfaces 49 physically connect the primary reflecting surfaces 48. The reflecting layer 46 is located on the primary reflecting surfaces 48 but can also be positioned on the shadow surfaces 49. A portion of the buffer layer 45 is between the incident surface 44 and the portion of the reflecting layer 46 on the shadow surfaces 49. Due to the angle at which the light signals are incident upon this portion of the buffer layer 45, this portion of the buffer layer 45 acts as a waveguide cladding and accordingly reduces interaction of the light signals with the metal on the shadow surfaces 49. As a result, this portion of the buffer layer 45 reduces absorption of the light from the free propagation region and further reduces the optical loss associated with the optical device.

As is evident from FIG. 1B, an upper portion of the reflecting layer 46 is positioned over the light-transmitting medium 18. For instance, a portion of the reflecting layer 46 is positioned such that the light-transmitting medium 18 is between the base 20 and the reflecting layer 46. Further, a portion of the free propagation region is between the base 20 and the reflecting layer 46. However, the buffer layer 45 is positioned between the upper portion of the reflecting layer and the light-transmitting medium 18. As a result, light signals in the free propagation region are not directly exposed to the upper portion of the reflecting layer 46. Accordingly, the light absorption associated with direct exposure of these light signals to the upper portion of the reflecting layer 46 is reduced.

The buffer layer 45 can be a different material from the first cladding 33 or can be the same material as the first cladding 33.

The thickness of the buffer layer 45 between the reflecting layer 46 and the incident surface 44 the can affect reflectivity at the reflecting surface. In some instances, the presence of the buffer layer results in a higher level of reflectivity at the reflecting surface than would occur if buffer layer were not present and the reflecting layer 46 and/or reflecting surface 47 were directly in contact with the light-transmitting medium 18. For instance, in some instances, the thickness of the buffer layer is selected such that the percentage of an incident light signal that is reflected at the reflecting surface is greater than would be without the buffer layer being present. A suitable thickness for the buffer layer 45 between the reflecting layer 46 and the incident surface 44 includes, but is not limited to, a thickness greater than 5 nm, or 10 nm or 15 nm and/or less than 50 nm, 100 nm, or 1000 nm. The thickness of an upper portion of the buffer layer 45 (the portion of the buffer layer between the top of the light-transmitting medium 18 and the upper portion of the reflecting layer 46) can affect amount of light signal absorption by the upper portion of the reflecting layer 46. For instance, reducing the thickness of the upper portion of the buffer layer 45 can increase light signal absorption. A suitable thickness for the upper portion of the buffer layer 45 includes, but is not limited to, a thickness greater than, 5 nm, or 10 nm or 15 nm and/or less than 50 nm, 100 nm, or 1000 nm.

The buffer layer 45 can have an index of refraction that is less than the index of refraction of the light-transmitting medium 18. As a result, the upper portion of the buffer layer 45 can act as a waveguide cladding or cladding of the free propagation region that reduces optical loss. Accordingly, in some instances, the first cladding 33 serves as the buffer layer 45 and is continuous with the buffer layer. For instance, the buffer layer 33 can extend from over the free propagation region and into the grating recess 42 such that the buffer layer 33 is positioned between the reflecting layer 46 and the incident surface 44. The use of the first cladding 33 as the buffer layer reduces the number of steps needed to fabricate the optical device by allowing both the buffer layer 45 and the first cladding 33 to be formed in a single step. In one example, the buffer layer 45, the insulator 22, and the first cladding 33 are each made of the same material. For instance, the buffer layer 45, the insulator 22, and the first cladding 33 can each be silica.

One or more secondary cladding layers can be optionally be formed over the portion of the optical device illustrated in FIG. 1A through FIG. 1D. For instance, FIG. 1E illustrates a secondary lading layer formed over the optical device of FIG. 1A through FIG. 1C. In particular, FIG. 1E illustrates the secondary lading layer formed over the portions of the optical device illustrated in FIG. 1B and FIG. 1C. Examples of secondary cladding layers include silica and silicon nitride. In one example, the one or more secondary cladding layers include a layer of silica and a layer of silicon nitride. The layer of silica can be between the optical device and the layer of silica nitride. In one instance, the layer of silica and the layer of silicon nitride are each about 1 micron thick.

FIG. 2A to 4C illustrate a method of forming an optical device according to FIG. 1A through 1C but with the first cladding 33 serving as the buffer layer 45 and being continuous with the buffer layer 45. The method is illustrated on a portion of a silicon-on-insulator chip; however, the method can also be executed using other platforms, chips, and/or wafers. Accordingly, the chip and/or wafer can serve as a device precursor. The method employs the self-alignment between the waveguides 26 and the optical grating 34 described in U.S. patent application Ser. No. 12/321,368, filed on Jan. 16, 2009, entitled “Optical Component Having Features Extending Different Depths into a Light Transmitting Medium,” and incorporated herein in its entirety.

FIG. 2A through FIG. 2C illustrate a portion of a device precursor upon which the waveguides 26 and optical grating 34 is formed. FIG. 2A is a topview of the device precursor. FIG. 2B is a cross section of the device precursor shown in FIG. 2A taken along the line labeled B in FIG. 2A. FIG. 2C is a cross section of the device precursor shown in FIG. 2A taken along the line labeled C in FIG. 2C.

A first mask 60 is formed on the device precursor. The first mask 60 leaves exposed a grating recess region of the device precursor and trench regions 64 of the device precursor. The grating recess region 62 is the region of the device precursor where the grating recess 42 is to be formed. The trench regions 64 are regions of the light-transmitting medium 18 where the trenches 28 are to be formed. The first mask 60 also protects a free propagation region 40 of the device precursor. The free propagation region 40 spans a gap between the trench regions 64 and the grating recess region 62. As will become evident below, the waveguide surfaces 30 and the incident surface 44 are etched sequentially. However, the first mask 60 will define the position of the waveguide surfaces 30 and the incident surface 44. Since a single mask defines the positions of the waveguides 26 and the optical grating 34, the alignment of the waveguides 26 and gratings that is present on the first mask 60 is transferred to the final device.

A suitable first mask 60 includes, but is not limited to, a hard mask such as a silica mask. A silica mask can be formed on the device precursor by thermally growing the silica on the top side of the device precursor followed by forming a photoresist on the silica. The photoresist can be formed with the pattern that is desired for the silica. The device precursor can then be etched and the photoresist removed to provide the patterned silica that serves as the first mask 60. Suitable etches include, but are not limited to, dry or wet etches and either isotropic or anisotropic etches.

A second mask 68 is formed on the device precursor of FIG. 2A through FIG. 2C as shown in FIG. 3A through FIG. 3C. FIG. 3A is a topview of the device precursor. FIG. 3B is a cross section of the device precursor shown in FIG. 3A taken along the line labeled B in FIG. 3A. FIG. 3C is a cross section of the device precursor shown in FIG. 3A taken along the line labeled C in FIG. 3C.

As is evident from FIG. 3A through FIG. 3C, the second mask 68 protects the grating recess region 62 of the device precursor while leaving the trench regions 64 exposed. The first mask 60 is left intact on the device precursor during the formation of the second mask 68. As a result, the regions of the device precursor that were originally protected by the first mask 60 remain protected by the first mask 60. Further, the second mask 68 is formed such that the first mask 60 continues to define the locations of the trench regions 64. As is evident form FIG. 3B, at least a portion of second mask 68 is positioned over the first mask 60.

After formation of the second mask 68, the waveguide surfaces 30 are formed by etching the device precursor to the depth that is desired for the trenches 28. For instance, the device precursor can be etched so as to provide the device precursor illustrate in FIG. 3A through FIG. 3C. As is most evident from FIG. 3C, the etch forms the trenches 28 and accordingly defines the locations of the waveguides 26 on the device precursor. Since the first mask 60 defines the locations of these trenches 28, the first mask 60 defines the locations of the waveguides 26. Suitable etches include, but are not limited to, dry or wet etches and either isotropic or anisotropic etches.

The second mask 68 is removed. After removal of the second mask 68, a third mask 70 is formed on the device precursor as shown in FIG. 4A through FIG. 4C. FIG. 4A is a topview of the device precursor. FIG. 4B is a cross section of the device precursor shown in FIG. 4A taken along the line labeled B in FIG. 4A. FIG. 4C is a cross section of the device precursor shown in FIG. 4A taken along the line labeled C in FIG. 4C.

As is evident from FIG. 4A through FIG. 4C, the third mask 70 protects the waveguides 26 while leaving the grating recess region 62 exposed. The first mask 60 is left intact on the device precursor during the formation of the third mask 70. As a result, the regions of the device precursor that were originally protected by the first mask 60 remain protected by the first mask 60. Further, the third mask 70 is configured such that the first mask 60 continues to define the location of the grating recess 42. As is evident form FIG. 4B, at least a portion of third mask 70 is positioned over the first mask 60.

After formation of the third mask 70, the device precursor is etched to the desired depth of the grating recess 42. For instance, the device precursor can etched so as to provide the device precursor illustrate in FIG. 4A through FIG. 4C. As is most evident from FIG. 4C, the etch forms the grating recess 42 and accordingly defines the locations of the grating on the device precursor. Since the first mask 60 defines the location of the grating recess 42 during the etch, the first mask 60 defines the locations of both the grating recess 42 and the waveguides 26 on the optical device. Suitable etches include, but are not limited to, dry or wet etches and either isotropic or anisotropic etches.

Suitable masks for the second mask 68 and/or the third mask 70 include, but are not limited to, photoresists. Other masks can be employed for the second and/or third mask 70. In some instances, the third mask is a hard mask. For instance, the third mask 70 can be generated from multiple layers as disclosed in U.S. patent application Ser. No. 12/321,368, filed on Jan. 16, 2009, entitled “Optical Component Having Features Extending Different Depths into a Light Transmitting Medium,” and incorporated herein in its entirety. Examples of other masks that may be employed for the second mask 68 and/or the third mask 70 include or consist of metals and/or polyimides. Examples of suitable metals include aluminum. The type of mask selected can be influenced by the processing parameters that depending on the type of etch being used or to enable a viable process flow for the integration of additional components onto the same substrate as discussed above. Examples of parameters that can influence mask selection include etch angle, level of smoothness desired for etched materials, etch selectivity and feature resolution.

Although FIG. 2A through FIG. 4C illustrate the waveguides 26 formed before the grating recess 42, the waveguides 26 and grating recess 42 can be formed in reverse order. For instance, the second mask 68 and the third mask 70 can be formed in reverse of the order disclosed above. As an example, the third mask 70 disclosed above can be formed before the second mask 68. Further, the third mask 70 can be formed and removed before the second mask 68 is formed.

The third mask 70 and the first mask 60 can be removed from the device precursor of FIG. 4A through FIG. 4C and the first cladding 33 can be formed on the light-transmitting medium 18 to provide the device precursor of FIG. 5A through FIG. 5C. FIG. 5A is a topview of the device precursor. FIG. 5B is a cross section of the device precursor shown in FIG. 5A taken along the line labeled B in FIG. 5A. FIG. 5C is a cross section of the device precursor shown in FIG. 5A taken along the line labeled C in FIG. 5C.

Suitable methods for forming the first cladding 33 on the light-transmitting medium 18 include, but are not limited to, depositing the first cladding 33 on the light-transmitting medium 18, growing the first cladding 33 on the light-transmitting medium 18. When the light-transmitting medium 18 is silicon and the first cladding 33 is silica, the silica can be thermally grown on the silicon by heating the device precursor in the presence of oxygen so as to convert a portion of the silicon to silica. Thermal growth of the first cladding 33 on the light-transmitting medium can be the preferred method of forming the first cladding 33 on the light-transmitting medium 18 since it results in the first cladding 33 following the shape of the incident surface 44 and accordingly, the stepped reflecting surfaces 47. In contrast, deposition methods can result in the first cladding 33 being formed with an uneven thickness on the incident surface 44.

The reflecting layer 46 is formed on the device precursor of FIG. 5A through FIG. 5C so as to provide the device precursor of FIG. 6A through FIG. 6C. FIG. 6A is a topview of the device precursor. FIG. 6B is a cross section of the device precursor shown in FIG. 6A taken along the line labeled B in FIG. 6A. FIG. 6C is a cross section of the device precursor shown in FIG. 6A taken along the line labeled C in FIG. 6C. FIG. 6A through FIG. 6C illustrate the optical device of FIG. 1A through FIG. 1C but with the first cladding 33 serving as the buffer layer 45 and being continuous with the buffer layer 45.

The reflecting layer 46 can be formed on first cladding 33 by evaporation, and sputtering. The reflecting layer 46 can then be patterned using traditional integrated circuit manufacturing techniques. For instance, the device precursor can be etched with a patterned fourth mask in place. A suitable fourth mask includes, but is not limited to, a photoresist and oxide. Suitable etches for use with the fourth mask include, but are not limited to, wet etches and dry etches.

One or more secondary cladding layers can optionally be formed over the portion of the optical device illustrated in FIG. 6A through FIG. 6C. Examples of secondary cladding layers include silica and silicon nitride. In one example, the one or more secondary cladding layers include a layer of silica and a layer of silicon nitride. The layer of silica can be between the optical device and the layer of silica nitride. Suitable methods for depositing the layer of silica on the optical device include, but are not limited to, plasma enhanced chemical deposition (PECVD). Suitable methods for depositing the layer of silicon nitride on the optical device include, but are not limited to, plasma enhanced chemical deposition (PECVD). In one example, the layer of silica and the layer of silicon nitride are sequentially deposited over the upper surface of the entire portion of the optical device illustrated in FIG. 6A through FIG. 6C. The layer of silica and the layer of silicon nitride can each be about 1 micron thick.

Although the optical above device is illustrated as having only waveguides, a free-space region, and an optical grating 34, only a portion of the optical device is illustrated. As a result, the optical device can include a variety of other components. In some instances, one or more of the waveguides 26 carry light signals to and/or from these other components. Examples of these components include, but are not limited to, demultiplexers, multiplexers, filters, switches, amplifiers, attenuators, lasers and other light sources, star couplers, and other waveguides. Additionally or alternately, the device can include electrical components. For instance, the device can include electrical connections for applying a potential or current to a waveguide and/or for controlling other features on the optical device.

Although the optical grating 34 is described as operating like a demultiplexer, the optical grating can be operated in reverse as a multiplexer. For instance, the output waveguides can be operated as input waveguides and the input waveguide can be operated as an output waveguide.

Other embodiments, combinations and modifications of this invention will occur readily to those of ordinary skill in the art in view of these teachings. Therefore, this invention is to be limited only by the following claims, which include all such embodiments and modifications when viewed in conjunction with the above specification and accompanying drawings. 

1. An optical device, comprising: a light-transmitting medium positioned on a base, the light-transmitting medium at least partially defining a free propagation region through which light signals travel; and a reflective grating positioned such that light signals that travel through the free propagation region can be received by the optical grating, the optical grating being configured to reflect the light signal back into the free propagation region, the optical grating reflecting the light signals such that the light signals associated with different wavelengths separate as the light signals travel through the free propagation region, a portion of the light-transmitting medium that defines the free propagation region having a facet through which the light signals are transmitted, the grating including a reflecting layer configured to reflect the light signals received by the grating, the grating including a buffer layer between the facet and the reflecting layer such that light signals transmitted through the facet and reflected by the reflecting surface travel through the buffer layer.
 2. The device of claim 1, wherein the buffer layer contacts the facet of the light-transmitting medium and the reflecting layer.
 3. The device of claim 1, wherein a cladding on the free propagation region serves as the buffer layer.
 4. The device of claim 1, wherein the light-transmitting medium at least partially defines waveguides through which light signals travel, the waveguide include an input waveguide and output waveguides, the optical grating is configured to reflect the light signal back into the free propagation region such that the light signals are received at the output waveguides and the optical grating reflects the light signals such that light signals associated with different wavelengths are received at different output waveguides, and a cladding for the waveguides serves as the buffer layer, the cladding contacting the light transmitting medium.
 5. The device of claim 4, wherein the cladding and the buffer layer are silica and the light-transmitting medium is silicon.
 6. The device of claim 1, wherein an upper portion of the reflecting layer is positioned on a top of the light transmitting medium and the facet extends from the top of the light transmitting medium toward the base.
 7. The device of claim 6, wherein the buffer layer is between the upper portion of the reflecting layer and the free propagation region of the light-transmitting medium.
 8. The device of claim 1, wherein the optical grating is an echelle grating.
 9. The device of claim 1, wherein the buffer layer is accordance with a layer that is thermally grown on the light-transmitting medium.
 10. The device of claim 1, wherein the reflecting layer includes a series of stepped reflecting surfaces, each reflecting surface including a primary reflecting surface and a shadow surface, the primary reflecting surfaces reflecting the light signals and being arranged such that the light signals associated with different wavelengths separate as the light signals travel through the free propagation region, the shadow surfaces physically connecting the primary reflecting surfaces, and the buffer layer and the reflecting layer being positioned on both the primary reflecting surfaces and the shadow surfaces.
 11. The device of claim 1, wherein the buffer layer is configured such that a percentage of the light signal reflected by the reflecting layer is greater than would occur if the buffer layer were not present between the facet and the reflecting layer.
 12. An optical device, comprising: a light-transmitting medium positioned on a base, the light-transmitting medium at least partially defining waveguides and a free propagation region through which light signals travel, the waveguide including an input waveguide and output waveguides; and a reflective grating positioned such that light signals guided by the input waveguide can travel through the free propagation region and be received by the optical grating, the optical grating being configured to reflect the light signal back into the free propagation region such that the light signals are received at the output waveguides, the optical grating reflecting the light signals such that the light signals associated with different wavelengths are received at different output waveguides, a portion of the light transmitting medium that defines the free propagation region having a facet through which the light signals are transmitted, the grating including a reflecting layer configured to reflect the light signals received by the grating, an upper portion of the reflecting layer being on a top of the light transmitting medium and the facet extends from the top of the light transmitting medium toward the base, and a buffer layer being between the upper portion of the reflecting layer and the free propagation region of the light-transmitting medium.
 13. The device of claim 12, wherein the buffer layer contacts the light-transmitting medium and the upper portion of the reflecting layer.
 14. The device of claim 12, wherein a cladding for the waveguides serves as the buffer layer, the cladding contacting the light-transmitting medium.
 15. The device of claim 14, wherein the cladding and the buffer layer are silica and the light-transmitting medium is silicon.
 16. The device of claim 12, wherein the buffer layer is arranged such that light signals that exit from the from the free propagation region of the light-transmitting medium and are reflected by the upper portion of the reflecting layer travel through the buffer layer. 